When fuels having low ignition quality (fuels having a low cetane number) are used in diesel engines, the must be replaced at low speed and at low load by a fuel having higher ignition quality. This has the particular drawback of considerably increasing engine operating costs.
To remedy this drawback, engines may be equipped with a device for pre-injecting a high ignition quality fuel, with pre-injection being used whenever engine running conditions no longer ensure proper combustion of low cetane number fuel. Such a device is described, for example, in U.S. Pat. No. 4,612,898 which shows a combustion chamber with a plurality of nozzles for injecting a low ignition quality main fuel together with an additional nozzle for injecting a high ignition quality fuel. Nevertheless, the low ignition quality fuel operating range guaranteed by presently known systems is inadequate.
The problem may be expressed as follows:
Operation of the engine at high load using a low ignition quality fuel that is relatively cheap is satisfactory.
If the load on the engine falls off progressively, there comes a point where proper combustion requires injection of an additional fuel having high ignition quality and which is expensive.
An object of the invention is to provide an ignition device making it possible to "delay" as much as possible the instant at which it becomes necessary to use pre-injection of a high ignition quality fuel, with the quantity of pre-injection fuel being kept as low as possible for reasons of cost. An object of the present invention is to provide an injection device enabling the engine to operate with low ignition quality fuel over a wider operating range.